code: def _flatten_columns(columns: list[tuple[str, str]]) -> list[str]:
      # Flatten the `MultiIndex` columns where names are concatenated with underscores.
      # Filtering is required to omit non-nested columns avoiding unwanted trailing underscores.
      return [
          "_".join(filter(lambda c: c, map(lambda c: str(c), col)))
          for col in columns
      ]
  
file_path: optuna/optuna/study/_dataframe.py
imports: from __future__ import annotations
  import collections
  from typing import Any
  from typing import DefaultDict
  from typing import Set
  import optuna
  from optuna._imports import try_import
  from optuna.trial._state import TrialState
label_desc: Use image node_id 2 for calling a global function with example usage: _flatten_columns(columns) and returns: unknown